1. Field of the Invention
This invention relates to an anti-skid control apparatus for a vehicle braking system which can prevent the locking of the wheels.
2. Description of the Prior Art
Such an anti-skid control apparatus for a vehicle braking system is known that includes fluid pressure control valve devices arranged between fluid pressure generating chambers of a tandem master cylinder and the wheel cylinders of the front wheels, respectively, and a control unit receiving outputs of wheel speed sensors for measuring or judging the skid conditions of the front and rear wheels and for generating instructions for controlling the fluid pressure control valve devices.
When the fluid pressure control valve device is provided for each of four wheels (four channels), and the fluid pressure of them are independently controlled, there is no problem on control operation. Or when the fluid pressure control valve device is provided for each of front wheels, and for both of rear wheels in common (three channels), there is no problem on control operation. In the latter case, the one common fluid pressure control valve device is controlled on the basis of the lower one of the speeds of the rear wheels.
However, in the above cases, three or four fluid pressure control valve devices are used. Accordingly, the whole anti-skid control apparatus is large-sized, and very heavy. Since the fluid pressure control valve device is expensive, it requires high cost.
Further, the brake pressures of the front and rear axles can be separately controlled from each other in the four or three channels. Accordingly when the automobile is of the so-called "four wheel drive type" (4WD) which includes at least a center differential as a torque distribution mechanism, there is the possibility that the phase difference and circulating torque occur between the brake controls of the front and rear axles. In that case, the driver feels uncomfortable.